This is the 9 round .22LR magazine for the historic Ruger Mk I (old style) pistol.
Manufactured from blued steel it has a full length thumb assist and is strippable for cleaning.
Note: the thumb assist button is removable and can be used on either side, for models with 'A-100' stamped on the left side the button is correct, for Mk I models without the 'A-100' mark the button should be swapped to the other side.